Thick film electrically resistive tracks

Abstract

In a thick film water track, irrespective of track thickness or the material of which the track is constructed, the optimum track width is found to be in the range of from 1.2 mm to 2.1 mm. Further, for a given resistance, the track is longer and may be conformed to a pattern to give improved temperature distribution. Additionally disclosed is a heating element having a number of thick film electrically resistive tracks applied to the surface of an electrically insulative substrate and a switch for selectively connecting one or more of the tracks to a power supply. The resistance and hence the operating temperature of the heating element may be varied by changing the track or tracks connected to the switch.

I claim: 
     
       1. A heating unit comprising an elongate thick film electrically resistive track of substantially constant width of between 1.2 and 2.1 millimeters along its length, to permit substantially even heat distribution over the length and width of the track, the length to width ratio being at least 10 to 1, the track being supported on a substrate of electrically insulative material, the track being configurated to achieve a predetermined heating profile. 
     
     
       2. A heating unit according to claim 1 wherein the width of said track is in the range 1.5 to 2.0 millimeters. 
     
     
       3. A heating unit according to claim 1 comprising a plurality of said tracks, said plurality of tracks being supported on said substrate of electrically insulative material and connected electrically in parallel with one another. 
     
     
       4. A heating unit according to claim 2 comprising a plurality of said tracks, said plurality of tracks being supported on said substrate of electrically insulative material and connected electrically in parallel with one another. 
     
     
       5. A heating unit according to claim 1 comprising a plurality of said thick film electrically resistive tracks supported on the surface of said electrically insulative substrate and switching means for selectively connecting one or more of said tracks to a power supply whereby the resistance and hence the operating temperature of said heating unit may be varied by changing the track or tracks connected to said switching means. 
     
     
       6. A heating unit according to claim 3 wherein each of said plurality of tracks has a different resistance. 
     
     
       7. A heating unit according to claim 4 wherein each of said plurality of tracks has a different resistance. 
     
     
       8. A heating unit according to claim 5 wherein each of said plurality of tracks has a different resistance. 
     
     
       9. A heating unit according to claim 6 wherein at least one of said plurality of tracks is made of a different material from the other tracks. 
     
     
       10. A heating unit according to claim 7 wherein at least one of said plurality of tracks is made of a different material from the other tracks. 
     
     
       11. A heating unit according to claim 8 wherein at least one of said plurality of tracks is made of a different material from the other tracks. 
     
     
       12. A heating unit according to claim 5 wherein at least one of said plurality of tracks is made of a material having in the range of from 0° C. to 550° C. a temperature coefficient of resistance in excess of 0.006 per degree C. 
     
     
       13. A heating unit according to claim 6 wherein at least one of said plurality of tracks is made of a material having in the range of from 0° C. to 550° C. a temperature coefficient of resistance in excess of 0.006 per degree C. 
     
     
       14. A heating unit according to claim 9 wherein at least one of said plurality of tracks is made of a material having in the range of from 0° C. to 550° C. a temperature coefficient of resistance in excess of 0.006 per degree C. 
     
     
       15. A heating unit according to claim 5, said operating temperature being determined by the resistance of a track connected to said switching means wherein said operating temperature may be altered by changing the track connected to said switching means. 
     
     
       16. A heating unit according to claim 6, said operating temperature being determined by the resistance of a track connected to said switching means wherein said operating temperature may be altered by changing the track connected to said switching means. 
     
     
       17. A heating unit according to claim 9, said operating temperature being determined by the resistance of a track connected to said switching means wherein said operating temperature may be altered by changing the track connected to said switching means. 
     
     
       18. A heating unit according to claim 12, said operating temperature being determined by the resistance of a track connected to said switching means wherein said operating temperature may be altered by changing the track connected to said switching means. 
     
     
       19. A heating unit according to claim 5, wherein said operating temperature may be altered by changing the number of tracks electrically connected in parallel to one another, said number of tracks being connected to said switching means. 
     
     
       20. A heating unit according to claim 12, wherein said operating temperature may be altered by changing the number of tracks electrically connected in parallel to one another, said number of tracks being connected to said switching means.